Cass and Erin are children of light and darkness. They will need both to rescue the Furies and make it out of the Underworld before it's too late. Finding out he was part Fury was weird enough, but now Cass is thrown into a battle he's not sure he wants to fight. He just wants to go back to his normal life, but the gods (and his sister, Erin) have other plans.
With the Furies imprisoned in Tartarus, Apollo...